After the apparitions’ warnings, Macbeth asks what is in the cauldron.

Macbeth is quite pleased with the warnings he gets from the three apparitions, because they seem to indicate that he is fairly safe. He should beware Macduff, but no man born of woman can hurt him and he won’t be vanquished until the forest comes to his caste. It seems like he’s in good shape.

Yet after the apparitions leave, Macbeth is still curious and asks the witches to show him one more thing.

I will be satisfied! Deny me this,

And an eternal curse fall on you! Let me know:

Why sinks that cauldron? and what noise is this? (Act 4, Scene 1)

What comes out of the cauldron is actually quite unnerving. It is Banquo’s line.

A show of eight Kings, and Banquo last with a glass in his hand. (4:1)

Macbeth asks what is going on, and the witches ask him why he is so surprised and leave.

This means that the original prophecy about Banquo’s sons being king is going to come true. It would seem that despite Macbeth’s efforts to wipe out Banquo, his sons are still going to be king and not Macbeth’s. Of course, Fleance did get away!